Re: [PATCH 4/4] xfs: report the correct read/write dio alignment for reflinked inodes

Darrick J. Wong
2025-01-06T17:33:12Z

On Mon, Jan 06, 2025 at 04:15:57PM +0100, Christoph Hellwig wrote:
> For I/O to reflinked blocks we always need to write an entire new
> file system block, and the code enforces the file system block alignment
> for the entire file if it has any reflinked blocks.
>
> Use the new STATX_DIO_READ_ALIGN flag to report the asymmetric read
> vs write alignments for reflinked files.
>
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
> fs/xfs/xfs_iops.c | 24 +++++++++++++++++++++---
> 1 file changed, 21 insertions(+), 3 deletions(-)
>
> diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
> index 6b0228a21617..053d05f5567d 100644
> --- a/fs/xfs/xfs_iops.c
> +++ b/fs/xfs/xfs_iops.c
> @@ -580,9 +580,27 @@ xfs_report_dioalign(
> struct xfs_buftarg *target = xfs_inode_buftarg(ip);
> struct block_device *bdev = target->bt_bdev;
>
> - stat->result_mask |= STATX_DIOALIGN;
> + stat->result_mask |= STATX_DIOALIGN | STATX_DIO_READ_ALIGN;
> stat->dio_mem_align = bdev_dma_alignment(bdev) + 1;
> - stat->dio_offset_align = bdev_logical_block_size(bdev);
> + stat->dio_read_offset_align = bdev_logical_block_size(bdev);
> +
> + /*
> + * On COW inodes we are forced to always rewrite an entire file system
> + * block or RT extent.
> + *
> + * Because applications assume they can do sector sized direct writes
> + * on XFS we fall back to buffered I/O for sub-block direct I/O in that
> + * case. Because that needs to copy the entire block into the buffer
> + * cache it is highly inefficient and can easily lead to page cache
> + * invalidation races.
> + *
> + * Tell applications to avoid this case by reporting the natively
> + * supported direct I/O read alignment.
> + */
> + if (xfs_is_cow_inode(ip))
> + stat->dio_offset_align = xfs_inode_alloc_unitsize(ip);
> + else
> + stat->dio_offset_align = stat->dio_read_offset_align;

This is a good improvement!
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>

--D

> }
>
> static void
> @@ -658,7 +676,7 @@ xfs_vn_getattr(
> stat->rdev = inode->i_rdev;
> break;
> case S_IFREG:
> - if (request_mask & STATX_DIOALIGN)
> + if (request_mask & (STATX_DIOALIGN | STATX_DIO_READ_ALIGN))
> xfs_report_dioalign(ip, stat);
> if (request_mask & STATX_WRITE_ATOMIC)
> xfs_report_atomic_write(ip, stat);
> --
> 2.45.2
>
>